1. Field of the Invention
The present invention relates to an apparatus for developing a latent image with a developer, and more particularly to a developing apparatus capable of defining the thickness of a toner layer on a toner supporting member in the presence of a magnetic field for using a carrierless one-component magnetic developer (hereinafter referred to as magnetic toner or simply as toner) in the image development.
2. Description of the Prior Art
The processes conventionally employed in the developing apparatus in the field of electrophotography and electrostatic recording can be classified into dry development and liquid development, and the former is further classified into processes utilizing a two-component developer and those utilizing a one-component developer. The two-component developing processes are differentiated by the carrier employed for transporting the toner particles and include magnetic brush development utilizing iron powder carrier, cascade development utilizing bed carrier, fur brush development utilizing a fur etc. Also the one-component developing process include powder cloud development utilizing suspended cloud of toner particles, contact or donor development in which the toner particles are brought into direct contact with the latent image bearing surface, toner transfer development in which the toner particles are not in direct contact with the latent image bearing surface but are made to fly thereto by the electric field generated by the electrostatic latent image, magnetic dry development in which magnetic electroconductive toner is brought into contact with the latent image bearing surface etc. The two-component developing processes, utilizing a mixed developer composed of carrier particles and toner particles, are inherently associated with a drawback of variation in the developed image density resulting from a change in the mixing ratio of said particles as the toner particles are consumed far faster than the carrier particles with the progress of the development, and also with a drawback of image quality deterioration resulting from the deterioration of difficultly consumable carrier particles after prolonged use thereof.
Among the one-component developing processes, the magne-dry development utilizing magnetic toner and the contact development not utilizing magnetic toner are defective in the tendecy of background fog formation resulting from the undifferential contact of the toner with the entire surface of both the image area and the non-image area. Such background fog formation is a defect also observable in the two-component developing processes. Also in the powder cloud development the background fog formation is inevitable as the deposition of powdered toner particles onto the non-image area is unavoidable.
In the field of the one-component developing processes there is also known a so-called toner transfer development in which the toner is applied uniformly on a bearing member such as a sheet and maintained in opposed relation with a small distance to the latent image bearing surface thereby attracting the toner from said bearing member to the latent image bearing surface by means of the electrostatic charge the latent image to effect the image development, as disclosed in the U.S. Pat. Nos. 2,839,400 and 3,232,190. This process is advantageous in reduced fog formation since the non-image area does not attract the toner nor is brought into contact with the toner. Also the process is alien from the above-mentioned variation in the mixing ratio or the deterioration of the carrier particles because of the absence of carrier particles.
Nevertheless this process has been associated with various difficulties as exemplified in the following:
(1) A uniform toner application on the toner supporting sheet is difficult to achieve although said sheet is in advance provided with an electric field to facilitate the toner application. The toner application for example with a known rigid blade is hardly capable of providing a thin and uniform particle layer, frequently leading to unevenness in the toner layer, which is directly reproduced in the developed image and is therefore not suitable for practical image reproduction. In order to avoid such drawback there has been proposed a process of utilizing paper or cloth on the surface of said toner bearing sheet and embedding the toner particles in the fibers constituting such paper or cloth, but such process does not necessarily ensure uniform application as it is difficult to prepare the toner particles finer than the fiber texture. Also the method of toner application on a bearing sheet in advance by cascade development method is not practical as it requires a large-sized apparatus.
(2) It is difficult to achieve uniform toner release from the toner bearing member. For uniform development a uniform toner release from thus applied toner layer to the image bearing surface is indispensable. Such toner release, being affected by the surface properties of the toner bearing sheet, the state of applied toner thereon and the characteristics of the toner particles, has never been improved to a practical acceptable level.
(3) A low resolving power.
In the conventionally known toner transfer development the toner is electrostatically deposited on the toner bearing member, and, even if it is made possible to form a relatively thin toner layer on said bearing member, the toner particles are released from said bearing member and fly toward the image bearing surface by the mutually repulsive charge of the toner particles when the distance to the image bearing surface becomes reduced approximately to 3 mm. However such wide clearance requires a long flight time for the toner particles from the bearing member therefor to the image bearing surface, and the flying particles are apt to be affected by the air flow in said clearance, the weight of the toner particles and the vibration of the image bearing surface or the toner bearing member, thus tending to result in an aberrated developed image. Also the electric field emanating from the electrostatic image of fine lines or fine-lined characters does not faithfully reach the toner bearing member, thereby resulting in a thinning of fine lines or fine-lined characters or in a significantly lowered resolution due to the lack of toner flight. On the other hand an excessively small clearance will result in a thickening of fine lines or fine-lined characters with loss of resolution, rendering it difficult to obtain faithful reproduction.
The present assignee already proposed, in the United States Patent Application Ser. No. 938,494, filed Aug. 31, 1978, a method of forming a uniform thin toner layer on the toner bearing member without the aforementioned difficulties, said method being featured by the use of a magnetic blade as a toner thickness limiting member and of a magnetic pole positioned in opposed relation to said blade, the toner layer on the toner supporting member being limited to a thin state by means of the magnetic field formed between said magnetic pole and said magnetic blade thereby enabling faithful image reproduction.
It has however been found that an excessively weak magnetic field between said magnetic blade and magnetic pole is unable to form a sufficiently thin toner layer, thus leading to a background fog or aberrated image due to enlarged thickness of the toner layer, while an excessively strong magnetic field provides an excessively thin toner layer, leading a thinning of image, particularly of line images.